1. Field of the Invention
This invention relates to a structure of a piezoelectric film which composes a piezoelectric film element. More particularly, this invention relates to a structure of a piezoelectric film element which is preferable for a drive source for discharging ink for an ink-jet recording head.
2. Description of the Related Art
An ink-jet recording head of an on-demand type comprises a piezoelectric film element which functions as a drive source for discharging ink. This piezoelectric film element comprises a piezoelectric film made of piezoelectric ceramics, and an upper electrode and a lower electrode to hold the piezoelectric film in between. The application of a desired electric field to the piezoelectric film element causes changes in volume, while the application of pressure causes changes in voltage. Since many piezoelectric ceramics with a perovskite crystal structure remarkably exhibit the above-described action, such piezoelectric ceramics are used as materials for the piezoelectric film. The piezoelectric film element provided with such a crystal structure is disclosed in, for example, Applied Physics Letters, 1991, Vol. 58, No. 11, p.p. 1161-1163. A prior art example of the ink-jet recording head using the piezoelectric film element is disclosed in, for example, a specification of U.S. Pat. No. 5,265,315.
Since the ink-jet recording head in these days are being required to achieve higher precision printing, the volume of a pressure room is getting smaller. In order to make the pressure room with the small volume discharge ink in an appropriate amount, it is necessary to cause higher pressure in the piezoelectric film element. This pressure is generated as accumulation of very little strains in individual crystal structures. Accordingly, it is supposed that as the thickness of the piezoelectric film is increased, higher pressure can be obtained. Moreover, by increasing the thickness of the piezoelectric film, it is possible to prevent the lowering of piezoelectric properties due to the generation of a strong electric field in the piezoelectric film. Therefore, attempts are being made to increase the thickness of the piezoelectric film by various film forming methods.
A piezoelectric film which is generally used has: a two-component composition containing lead zirconate titanate (hereinafter sometimes referred to as xe2x80x9cPZTxe2x80x9d) as a principal component; or a three-component composition prepared by adding a third component of PZT to the two-component composition. As a method for forming the piezoelectric film, for example, xe2x80x9cJOURNAL OF APPLIED PHYSICSxe2x80x9d (Vol. 83, Number 4, Feb. 15, 1998, p.p. 2202-2208) discloses the technique to form, by a sol-gel method, a film of Pb(Zr0.30Ti0.70)O3(PZT30/70) over an electrode made of platinum/titanium in the temperature environment of 510xc2x0 C.
This sol-gel method is the method of giving dehydration treatment to a hydrate complex (sol) of a hydroxide of a metal component of a PZT type piezoelectric film to turn it into a gel and of heating and burning the gel to adjust an inorganic oxide (piezoelectric film). This method makes it possible to form the film by repeating the coating, drying and pyrolyzing of a precursor of the PZT type piezoelectric film for several times until a specified thickness is obtained. Accordingly, this method is excellent in the composition control and is preferred for the adjustment of the thickness of the piezoelectric film. Moreover, patterning using a photoetching step is also possible and has actually been applied to an ink-jet recording head.
For example, when a PZT film with a thickness of about 0.4 xcexcm is to be formed by the sol-gel method, the step of spin coating, drying and pyrolyzing a sol for the PZT film is repeated for several times (for example, four times) and the step of RTA thermal treatment (final annealing) is then taken, thereby obtaining the desired PZT film.
However, the inventors of this invention have found that if the piezoelectric film is formed by the above-described sol-gel method and if an attempt is made to increase the film thickness to a certain degree, residual stress affects the inside of the piezoelectric film and cracks may be sometimes generated on the surface of the film when crystal grains are caused to grow in the pyrolyzing step and the RTA step. It is assumed that this phenomenon is caused because heat stresses act upon each other in a complicated manner when a molecular structure which is in the amorphous state in the pyrolyzing step or the RTA step of the piezoelectric film is turned into a minute crystal structure. Accordingly, it has been impossible to form a piezoelectric film with a large film thickness beyond a certain degree and technical limitations have been imposed upon the achievement of high precision of the ink-jet recording head.
It is an object of this invention to provide a piezoelectric film element having structural characteristics that make it possible to prevent the generation of cracks in the step of manufacturing the piezoelectric film element and to increase the thickness of a piezoelectric film, and also to provide a method for manufacturing such a piezoelectric film element. It is another object of this invention to provide an ink-jet recording head which has excellent ink discharging properties and to provide a method for manufacturing such an ink-jet recording head. It is a further object of this invention to provide an ink-jet printer which has excellent ink discharging properties.
In order to attain the above-described objects, a piezoelectric film element of this invention comprises a dislocation layer in a piezoelectric film. This xe2x80x9cdislocation layerxe2x80x9d is the layer of which atoms in crystals are partly defective, which is caused by lattice defects. It is desirable that a dislocation density in the dislocation layer be within the range of 1013/cm2 to 1014/cm2. This is because the structural strength of the piezoelectric film lowers if the dislocation density is more than 1014/cm2, while stresses caused within the film may not be relaxed if the dislocation density is less than 1013/cm2. It is also desirable that the dislocation layer be formed with a thickness ranging from 5 nm to 15 nm, more preferably 10 nm, in a thickness direction of the piezoelectric film. This is because the stresses caused in the film may not be relaxed if the thickness is less than 5 nm, while the structural strength of the piezoelectric film lowers if the thickness is more than 15 nm. It is particularly desirable that the piezoelectric film have a plurality of dislocation layers in its film thickness direction and that the dislocation layers be formed in a manner such that the distances between the adjacent dislocation layers are the same or gradually become shorter from the lower electrode side to the upper electrode side. As stresses caused at the time of formation of the piezoelectric film strongly affect the surface of the piezoelectric film (on the upper electrode side), the above-described structure causes the piezoelectric film to contain many dislocation layers in the vicinity of the surface of the piezoelectric film. Accordingly, the stresses caused at the time of formation of the piezoelectric film can be effectively relaxed. Therefore, it is possible to increase the thickness of the piezoelectric film. Moreover, it is possible to improve the yield of the piezoelectric film element, thereby reducing manufacturing costs.
The piezoelectric film element of this invention can be manufactured by a sol-gel method. Specifically speaking, the step of giving thermal treatment to a first film formed with a sol for forming a piezoelectric film precursor applied not less than once, and then giving thermal treatment to a second film formed with the sol applied over the first film not less than once, thereby forming a dislocation layer in the second film in the vicinity of an interface between the first film and the second film, is repeated for m times, and the m dislocation layers are formed in the film thickness direction of the piezoelectric film in a manner such that the distances between the adjacent dislocation layers are the same or gradually become shorter from the lower electrode side to the upper electrode side. As a result of the thermal treatment (pre-annealing) of the first film, the lattice arrangement over the surfaces of crystal grains of the sol is disturbed moderately when the crystal grains grow. Accordingly, the dislocation layer can be formed in the second film. When a plurality of dislocation layers are to be formed in the piezoelectric film, the step of applying the sol and the step of thermal treatment should be repeated more than once.
The piezoelectric film element of this invention can be used for an ink-jet printer which obtains desired visible images by selectively causing ink drops to be spattered and fixed over recording paper in accordance with print data to be inputted. An ink-jet recording head of this invention comprises: a pressure room substrate including a pressure room; and a piezoelectric film element of this invention as located at a position which makes it possible to press the pressure room. A method for manufacturing an ink-jet recording head of this invention comprises the steps of: forming a pressure room in a pressure room substrate; and manufacturing a piezoelectric film element at a position which makes it possible to press the pressure room by the method for manufacturing a piezoelectric film element of this invention.
An ink-jet printer of this invention comprises: a paper feed for feeding printing paper; an ink-jet recording head of this invention; a drive mechanism for scanning the ink-jet recording head over the paper; a storage means for storing print information; and a controller for reading print information from the storage means and for controlling the ink discharge of the ink-jet recording head and the scanning of the drive mechanism.